Where is the I Marine Expeditionary Force primarily based?

The I Marine Expeditionary Force (I MEF) is primarily based in Southern California and Arizona, making this the correct answer. This force is headquartered at Marine Corps Base Camp Pendleton, located in northern San Diego County, California. Camp Pendleton is one of the largest Marine Corps bases in the United States and serves as a major hub for training and operational readiness.

In addition to Camp Pendleton, I MEF also encompasses other installations, including Marine Corps Air Station Miramar in San Diego, CA, and various training areas that extend into Arizona. This geographical location provides the I MEF with access to diverse training environments and rapid deployment capabilities, vital for its mission to conduct amphibious operations and support expeditionary warfare.

The other options reflect regions where different Marine Corps units are located or do not represent the primary base of the I MEF, which is firmly established in the Southern California and Arizona areas.